misc/ncnn: fix build without libomp
CMake Error at /usr/local/share/cmake/Modules/FindOpenMP.cmake:261 (try_compile):
Failed to generate test project build system.
Call Stack (most recent call first):
/usr/local/share/cmake/Modules/FindOpenMP.cmake:537 (_OPENMP_GET_FLAGS) src/CMakeLists.txt:205 (find_package)